The investigation of physical factors affecting biological experiments was continued. These experiments included both the radiobiological effects on different inbred strains of mice and the effect of various physical factors on the success of bone marrow transplantation experiments. Physical factors, heretofore, considered of little consequence when dealing with biological material have been demonstrated to be critical for the reproducibility of these experiments in other laboratories. New parameters for "defining" x-ray treatment are required along with more precise reporting of experimental conditions.